The application of velocity control to transmitting piezoelectric transducers is extremely difficult because of the inherent instability of piezoelectric materials. The properties of these materials change in a complex manner when under the influence of time, temperature and pressure. When assembled into a transducer, there is additional variability associated with components that can cause localized heating in the joints between the piezo elements. Velocity control can be used to improve the performance of transducers that are used in a variety of applications, including high power medical applications (such as, cataract fragmentation, kidney stone fragmentation, liposuction, suture welding, and thrombi ablation), and dental, industrial cleaning, and sonar applications. Transducers used for high power medical applications are usually referred to as handpieces.
Sonar transducers are usually assembled into a multi-element array in order to improve or modify the directional response of a single transducer. Variations in the piezo properties of individual transducers within an array can result in variations in the relationship between drive current and the velocity of the radiating surface. The directional response of a single transducer and an array of transducers are characterized by the formation of a beam in a preferred direction and a number of lower intensity side lobes. An array of transducers can be mechanically steered to a preferred direction or it can be electrically steered by applying phase or time delays to the individual transducers.
When the array of transducers is driven at high power, the piezo material within the transducers in the center of the array will increase in temperature to a greater degree than those disposed around the outside of the array. Therefore under long-term operation the effective coupling coefficient, k, of the transducers in an array will be reduced in a non-uniform manner that will increase the level of the side lobes and degrade the directional performance of the array. Also, there is a variation in the effective coupling coefficient of sonar transducers that is associated with manufacturing tolerances.
There is therefore a need in the art to apply velocity control in a manner that will compensate for variation in the effective coupling coefficient of the transducers within an array. By applying velocity control to the individual transducers within the array, the level of the side lobe intensities can be reduced and thus improve the directional discrimination of the main beam. The side lobe level can be reduced to very low levels by a technique known as amplitude shading whereby the velocity of individual transducers in the region of the center of the array are greater than those of transducers located at the edge of the array.
The need for effective or enhanced velocity control is most acute for high power endoscopic medical procedures where the precise control of cutting, fragmentation or stress-generated heat is critical. It is therefore important that a power level setting on the handpiece control instrument corresponds with a specific value of end effector velocity. For procedures where the operative site can be directly viewed, such as cataract fragmentation and teeth cleaning, velocity control is achieved by a variable foot peddle and automatic human feedback. However, these handpieces need to be automatically characterized at high power prior to use and the velocity needs to be controlled during this tune cycle. The prior art ultrasonic generator systems have little flexibility with regard to amplitude control because of unpredictable changes in the handpiece electro-mechanical characteristics caused by component tolerances, assembly method, and environmental conditions. These changes primarily result in variations in the stored electrical energy within the transducer. Therefore, the effective coupling coefficient, k, will change since this parameter is defined as the square root of the ratio of the mechanical stored energy to the total input energy. The impedance at resonance is inversely proportional to the effective coupling of the transducer. Thus, for a constant value of current, increasing the value of the coupling coefficient will result in less radiated and/or dissipated power and reduced tip/end effector displacement. Conversely, a reduction in the value of the coupling coefficient will result in higher impedance at resonance and increased power, voltage, and tip displacement. As most transducer control systems assume a linear relationship between current and tip velocity, decreases in the value of the effective coupling coefficient can result in high operational voltage and tensile failure in highly stressed components. These failures are most likely to occur during the tune cycle prior to actual use where the control system typically characterizes the handpiece at a higher power level.
U.S. Pat. No. 6,678,621 to Wiener, et al. describes a method of output displacement control using phase margin in an ultrasonic scalpel handpiece. Prior to operational use, an ultrasonic surgical handpiece is calibrated by causing it to be driven with an output displacement that is correlated with the phase margin, which is the difference of the resonant frequency and the anti-resonant frequency of the handpiece. A frequency sweep is conducted to find the resonant frequency and the anti-resonant frequency for the handpiece. The resonant frequency is measured at a point during the frequency sweep where the impedance of the handpiece is at its minimum. The anti-resonant frequency is measured at a point during the frequency sweep where the impedance of the handpiece is at its maximum. Using a target or specific output displacement, a drive current is calculated based on the phase margin. The handpiece is then controlled by the current output from a generator console to provide a given output displacement. To ensure these measurements are accurate and not effected by secondary resonances, the initial test data is stored in a micro-chip that is embedded within the transducer or the transducer connector. Complex adaptive control algorithms adjust the generator output current to maintain consistent velocity at the distal tip of the end effector.
Although simple in concept, this is a relatively complex method to implement as a practical system control algorithm, because it involves multiple measurements of impedance during the frequency sweep. It also involves a calculation based on the subsequent detection of both a maximum and minimum value of impedance. Typically, the number of measurements would be in the range of 100 to 5000 and would take a few seconds. Applying the method while a transducer is operating at full power would therefore result in an unacceptable interruption to the function of the end effector during the acquisition of impedance data.
Detecting secondary resonances, as shown in the measured data in FIG. 1, would also not be practical while the transducer is operational. For example, the frequency sweep data would need to be compared with the data stored in the micro-chip and this would take additional time. Secondary resonances are often caused by the attachment screw of the end effector. The application of ultrasonic energy tends to loosen the screw and this may not be detected during a calibration procedure prior to operational use.
There is therefore a general need in the art for a simplified method of controlling the transducer output current to achieve a desired value of end effector or radiating surface velocity that does not involve the use of an embedded micro-chip. There is also a need for a control method that can be implemented both prior to and during operational use and can be universally applied to both ultrasonic and sonar transducers.